This invention relates to a motorized power steering apparatus for automobiles or other vehicles. In the past, power steering apparatuses for automobiles were usually driven by hydraulic power generated by a pump. However, as hydraulic systems are bulky and heavy, in recent years, there has been a trend towards the use of electric motors to provide the drive force for power steering. A power steering apparatus which employs an electric motor to generate an auxiliary torque to assist the steering of the vehicle is referred to as a motorized power steering apparatus.
In a motorized power steering apparatus, a torque sensor measures the steering torque applied by the driver to a steering wheel. An electric motor which is connected to a suitable portion of the steering gear is then controlled in accordance with the measured torque to impart an auxiliary steering force to the steering gear. The auxiliary steering force lessens the force which need be applied to the steering wheel by the driver.
Generally, as the speed of a vehicle increases, the resistance between the road surface and the tires decreases, so the force required for steering the vehicle also decreases. If the auxiliary steering force which is generated by the electric motor were the same at high and low speeds, the resistance to steering felt by the driver would become too light at high speeds. If the driver exerted a sudden torque on the steering wheel at a high vehicle speed, the wheels of the vehicle could turn too sharply, possibly upsetting the stability of the vehicle.
Therefore, in a motorized power steering apparatus, the electric motor is controlled so that the torque which it outputs in response to a given steering torque is lower at high vehicle speeds than at low vehicle speeds. In many conventional power steering apparatuses, the motor is controlled by a microcomputer equipped with an internal memory in which is stored a table of data relating the motor output to the steering torque for all vehicle speeds. When the microcomputer receives signals indicating the vehicle speed and the steering torque applied by the driver, based on the data in the memory, it calculates the appropriate amount of torque to be generated by the motor and controls the motor current accordingly.
In order to perform fine, continuous control of the electric motor over the entire speed range of the vehicle, a large amount of data is necessary. The internal memory of the microcomputer must have a large capacity to accommodate all the data, and it therefore tends to be expensive. Taking everything into consideration, conventional motorized power steering apparatuses which employ a microcomputer are complicated, unreliable, and costly.